1. Field of the Invention
This invention relates to a manufacturing method for a coil spring used in, for example, a suspension mechanism of a vehicle, and more particularly, to shot peening conditions.
2. Description of the Related Art
It is conventionally known that the fatigue strength of a coil spring can be improved by applying compressive residual stress to the vicinity of the surface of the spring by shot peening. Multistage shot peening is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-345238 or Jpn. Pat. Appln. KOKAI Publication No. 2008-106365. In the multistage shot peening, a plurality of shot peening cycles are performed separately. Further, stress peening and warm peening (hot peening) are also known as means for producing compressive residual stress in a region ranging from the surface of the spring to a deep region. In the stress peening, the coil spring is compressed as a shot is projected. In the warm peening, the coil spring is heated to a temperature of about 250° C. as a shot is projected.
The stress peening requires equipment for compressing the coil spring. Since the coil spring is compressed as the shot is projected, moreover, the intervals between the turns of the spring wire become shorter. Accordingly, there is a problem that shots cannot be easily applied to the inside of the coil spring or between the spring wire turns. In the warm peening, a desired residual stress distribution cannot be obtained unless the temperature is appropriately maintained, so that temperature control is difficult.
Possibly, on the other hand, the fatigue strength of the coil spring may be improved by adding a specific alloy component to spring steel. However, spring steel containing a specific alloy component is expensive and causes an increase in the cost of the coil spring.